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Stage 3 LarvaeWith the notable exceptions of ascarids and pinworms, most nematodes become infective during the third larval stage (L3) after the second molt. While it is during this period that they infect the host where they will eventually reproduce, different nematodes go about it in different ways. In some instances, the host may become infected by accidentally swallowing the larvae. Other stage 3 larvae, like those of the hookworm, directly invade the host. Filarial worms, on the other hand, are again transferred by fly bite from the intermediate host to the final host. Finally, Trichinela spiralis larvae remain in their initial host, go into dormancy in the muscles during stage 3, and then infect a new host when it eats the contaminated meat. MigrationOnce inside the final, or definitive, host, nematode larvae molt another two times until they develop into immature adults after four larval stages. They also migrate through the host's body, using the bloodstream or lymphatic system. The larvae travel from the heart to the lungs, trachea and intestine. ReproductionNemotodes usually require two different sexes to reproduce and, therefore, develop into both males and females as adults. The males produce sperm in the testis and deposit them into receptacles in the female. The female holds onto the sperm until she needs to use them for fertilization. After she has finished fertilization, the eggs develop in her uterus. When they are ready, she then lays them in the host, using her muscular ovijector. At this point, the life cycle of nematodes is complete.  
Phylum Annelida Reproductive OrgansWith the exception of the marine polychaetes, annelids are hermaphrodites: Each individual has male and female reproductive organs. Still, an annelid cannot reproduce without a contribution from a mate. Polychaete worms are either male or female. Egg LayingEarthworms, perhaps the most familiar annelids, mate before laying eggs. Two worms bind themselves to each other while each worm passes a sperm packet to the other. After mating, the broad, saddle-like band on the worm (called the clitellum) secretes a mucus sheath that begins to move toward the head of the worm. As it moves forward, the worm secretes sperm and eggs into the sheath, which eventually forms an egg cocoon. Terrestrial annelids lay their eggs in the soil, whereas aquatic annelids deposit or attach their egg cocoon to plants or to the soil substrate. Marine polychaetes transform into a reproductive stage called an epitoke before mating. The polychaete's male and female epitokes release sperm and eggs into the water. Larval StageMarine polychaetes have a free-living larval stage, called a "trochophore." The trochophore eventually transforms into the adult form. Adult HabitatsNewly hatched or metamorphosed annelids settle into adult habitats. Most adult annelids live in the soil. Marine polychaetes live in the soil substrate of their aquatic habitat. Some marine polychaetes create tubes in the mud, and these somewhat rigid tubes provide protection. Other parasitic annelids are free-living. Adult EcologyMost adult annelids ingest soil, digest organic nutrients and excrete the inorganic leftovers---sand particles, for example. Some parasitic species such as leeches, however, feed on other organisms. A few species even prey on other invertebrates.  
 
Figure 7-31 Larval types of digenetic flukes: A, Miracidium. B, Sporocyst. C, Redia. D and E, Cercariae. F, Metacercaria (From the U.S. Naval Medical School Laboratory Manual.)  Figure 7-38 Some stages in the lite cycles of the fish tapeworm, Diphyiiobothrium latum: A, Ciliated oncosphere. (After Vcrgcer.) B, Mature- procercoid. (After Brumpt.)
Lecture # 10. General characteristics of the type of Arthropods.
1. Characteristic features of the structure and development. Key members of their distribution. Arthropods are a vast assemblage of animals. At least three quarters of a million species have been described; this is more than three times the number of all other animal species combined (see figure inside cover). The tremendous adaptive diversity of arthropods has enabled them to survive in virtually every habitat; they are perhaps the most successful of all the invaders of the terrestrial habitat. Arthropods represent the culmination of evolutionary development in the protostomes. They arose either from primitive stocks of polychaetes or from ancestors common to both, and the relationship between arthropods and annelids is displayed in several ways. 1 Arthropods, like annelids, are metameric. Metamerism is evident in the embryonic development of all arthropods and is a conspicuous feature of many adults, especially the more primitive species. Within many arthropod groups there has been a tendency for metamerism to become reduced. In such forms as mites, for example, it has almost disappeared. Loss of metamerism has occurred in three ways. Segment become lost, segments have become fused together, and segmental structures, such as ц ages, have become structurally and functio. ferentiated from their counterparts on segments. Different structures having the same segmental origin arc said to be serially homologous. Thus, the second antennae of a crab are serially homologous to the chclipeds (claws), for evolved from originally similar segmental appendages. 2 In the primitive condition each arthropod segment bears a pair of appendages. This same condition is displayed by the polychaetes, in which each metamere bears a parapodia. However, the homology between pi podia and arthropod appendages is uncertain. 3 The nervous systems in both groups are constructed on the same basic plan. In both a dorsal anterior brain is followed by a ventral nerve cod containing ganglionic swellings in each segment. 4 The embryonic development of a few pods still displays holoblastic determinate dermage, with the mesoderm in these forms arising from the 4d blastomere. Exoskeleton Although arthropods display these annelidan characteristics, they have undergone a great many profound and distinctive changes in the course of their evolution. The distinguishing feature of arthropods, and one to which many other changes are related, is the chitinous exoskeleton, or cuticle, that covers the entire body. Movement is made possible by the division of the cuticle into separate plates. Primitively, these plates are confined to segments, and the plate of one segment is connected to the plate of the adjoining segment by means of an articular membrane, a region in which the cuticle is very thin and flexible. Basically, the cuticle of each segment is divided into four primary plates—a dorsal tergum, two lateral pleura, and a ventral sternum (Fig. 1). This pattern has frequently disappeared because of either secondary fusion or subdivision. 
Fig 1 , Cross section. The cuticular skeleton of the appendages, like that of the body, has been divided into tubelike segments, or sections, connected to one another by articular membranes, thus creating a joint at each junction. Such joints enable the segments of the appendages, as well as those of the body, to move (hence the name of the phylum, Arthropoda— jointed feet). In most arthropods the articular membrane between body segments is folded beneath the anterior segment. In| arthropods the additional development condyles and sockets is suggestive of ventral skeletal structures. In addition to the external skeleton, then in also been the development of what is called the endoskelcton. This may be ail of the procuticle that produces inner apodemes, to which the muscles, or it may involve the sclerotizing internal tissue, forming free plates for muscle attachment within the body. The arthropod skeleton is secreted by the derlying layer of integumentary epithet known as the hypodermis. It is composed of outer epicutiece and a much thicker procuticle (Fig 2).
I ftecolorof arthropods commonly results from bdtposition of brown, yellow, orange, and red ^pigments within the cuticle. However, іг-fctot greens, purples, and other colors result bfestriationsof the epieutiele, which caustic refaction and give the appearance of color. Ы,body coloration docs not originate directly cuticle but instead is produced by subcuti-thromatophores or is caused by blood and tis-ipigments, which arc visible through a thin, (cuticle. Despite its locomotor and supporting advantages, an external skeleton poses problems for a growing animal. The solution evolved by the arthropods has been the periodic shedding of the skeleton, a process called molting or ecdysis. Before the old skeleton is shed, the epidermal layer (hypodermis) secretes proenzymes (inactive enzymes) at the base of the skeleton. The hypodermis now detaches from the skeleton, a process referred to as apolysis, and secretes a new epieutiele or at least its outer cuticulin layer (Fig. 12-4B). The proenzymes secreted earlier—chitinase and protease—become activated and digest the un-tanned endocuticle (Fig. 12-4C). The products of digestion are reabsorbed through the new cuticulin envelope. With the erosion of the old endocuticle, the hypodermis secretes new procuticle. At the ultrastructural level there are only three components to the skeleton. The outer cuticulin envelope and chitin fibers, the latter forming most of the arthropod skeleton, are laid down by plasma membrane plaques of the hypodermis (Fig. 12-4E). These two skeletal components are separated by the third component, proteins, which are deposited by exocytosis (Locke, 1984). At this point the animal is encased within both an old and a new skeleton (Fig. 12-4D). The old skeleton now splits along certain predetermined lines and the animal pulls out of the old encase- ment. The new skeleton is soft and commonly wrinkled and is stretched to accommodate the increased size of the animal. Stretching is brought about by blood pressure, facilitated by the uptake of water or air by the animal. Hardening of the cuticle results from tanning of the protein and from stretching. Additional procuticle may be added following ccdysis, and in some arthropods, such as insects, additions are made to the epicuticle by secretions through the pore canals. The final surface of the epicuticle is often formed by a cement layer. Sensory structures and muscle connections pose special problems for the molting process. Sensory structures, such as hairs, are laid down bena old skeleton, usually horizontally against 4 skeleton. The dendrite may retain connectia the old hair until broken at ecdysis. Muscles are attached to the exoskcletonl crotubules in specialized epidermal cells. Tj crotubules are anchored to an internal fold exoskeleton containing a fiber that runs all tl to the epicuticle (Fig. 12-5). The fiber is J gested during the molting process and mainl connection between the old and new site until severed at ecdysis. The stages between molts are known asii and the length of the instars becomes longer »mes older. Some arthropods, such as almost crabs, continue to molt through-Ik Other arthropods, such as insects and lm more or less fixed numbers of instars, ring attained with sexual maturity. Oghanarthropod is measurably larger and llowingecdysis (Fig. 12 -6), growth is ac-nuous, as in most other animals. Proper organic compounds arc synthesized lintermolt period, replacing fluids taken ing is under hormonal control. Ecdvsonc, Iky certain endocrine glands (tor example, iracic glands in insects), is circulated by ■Beam acts directly on the epidermal cells. The production of ecdysone is in turn regulated by other hormones. Although most studied and best understood in insects and crustaceans, ecdysone controls molting in all arthropods. Molting physiology will be described in some detail for crustaceans Movement and Musculature As movement in arthropods has become restricted to flexion between plates and cylinders of the cuticle, a related change has taken place in the nature of the body musculature. In annelids the muscles take the form of longitudinal and circular sheathlike layers of fibers lying beneath the epidermis. Contraction of the two layers exerts force on the coelomic fluid, which then functions as a hydrostatic skeleton. In arthropods, on the other hand, these muscular cylinders have become broken up into striated muscle bundles, which are attached to the inner surface of the skeletal system (Figs. 12-1B and 12-2). The muscles are attached to the inner side of the exoskeleton by specialized hypodermal cells (Fig. 12-5). Flexion and extension between plates are effected by the contraction of these muscles, with muscles and cuticle acting together as a lever system. This со functioning of the muscular system and skeletal system to bring about locomotion is similar to that in vertebrates. Extension, particularly of the appendages, is accomplished, in part or entirely, by an increase in blood pressure. Arthropods employ as their chief means of locomotion jointed appendages, which act either as paddles in aquatic species or as legs in terrestrial groups. Our knowledge of arthropod locomotion, especially locomotion on land, results largely from the extensive studies of Manton (1978). In contrast to the parapodia of polychaetes, the locomotor appendages of arthropods tend to be more slender, longer, and located more ventrally. Despite the more ventral position of the legs, the body usually sags between the limbs (Fig. 12-7A). In the cycle of movement of a particular leg, the effective step, or stroke, during which the end of the leg is in contact with the substratum, is closer to the body than the recovery stroke, when the leg is lifted and swung forward (Fig. 12-7B to £). Among the several factors determining speed of movement, the length of stride is of obvious importance, and stride lop increases with the length of the leg. The ргоЫя of mechanical interference are decreased by duction in the number of legs to five, four, or tl pairs and by differences in leg length and therij tive placement of the leg tip. In arthropods thi have retained a large number of legs, suchasel tipedes, the fields of movement of individual! overlap those of other legs (Fig. 12-7BL Ford* animals the difference in proximity of the lepl the body during the effective and recovery stroke prevents mechanical interference. The arthropodan gait involves a wave of lot movement, in which a posterior leg isputdfl^ just before or a little after the anterior leg is lil The movements of legs on opposite sides ofj body alternate with one another; i.e., one pair is moving through its effective strokewhilei| mate is making a recovery stroke. AlternateЦ movement tends to induce body undulation.! tendency is counteracted by increased bodyi\ ity, such as the fused leg-bearing segmentsd form the thorax and cephalothoraxof insccts,H crustaceans, and arachnids. An exoskeleton makes a highly efficient! motor-skeletal system for animals that are onhil few centimeters long. It provides protection red dition to support, and there is a large surfaced for the attachment of muscles The tubular ad struction resists bending. However, the walla buckle on impact if there is insufficient skekd material, just as you cannot bend a cvlindricaleJ but you can buckle one wall by kicking it. Тһим| exoskeleton imposes limits on the maximum! of arthropods. The weight of a large animalandtkj resulting stress produced when moving wouldd quire heavy skeletal walls. But when theartbT molted, the soft, new skeleton would collaj under the animal's weight before hardeningoj occur. Significantly, the largest arthropods livtil the sea, where the aeiuatie medium providesraT more support than air. In contrast to the condition in vcrtebrates,d arthropod muscle contains relatively few fibend is innervated by only a small number of neun Many axon terminals arc provided to one mud fiber (Fig. 12-8), and one neuron may supply m than one muscle. Moreover, several typesofm) neurons—phasic (fast) neurons, tonic islowldj rons, and inhibitory neurons may supplvasindj muscle. The terms phasic and tonic, or fasti slow, refer not to the speed of transmissionbtnj the nature of the muscle response. The impulK phasic motor neurons produce rapid but briefco*
Tie neuromuscular system may be further implicated, as in at least the crustaceans, by the ttrentiationofthe muscle fibers into phasic and Wtypes,each having a distinctive ultrastructure ffldphvsiclogy. Some muscles arc entirely phasic, ■rare entirely tonic, and some are mixed. Phasic Iworneurons innervate only phasic muscle fibers; \m motor neurons innervate both phasic and Lib or, in some instances, only tome fibers. la vertebrates graded responses depend in large I pen die number of motor units contracting. Ar-^muscles are not organized as motor units, awever. and graded responses depend on the type (muscle fibers contracting, the type ot neuron Wandthe interaction of different types of ncu-■ №example, two different extensor muscles Kinnervated by the same motor fiber in a crayfish dw.but the two muscles function independently Leeach is innervated by separate inhibitory ■ras. The organization of arthropod ganglia is like Moi annelids and mollusks described on page 21.Giant fiber systems are frequently well devcl-ajd,and "command" systems have been identi-U.Arthropod neural networks and neuromuscu-isystems have been best studied in crustaceans. 'Bltsted students should consult Kennedy et al (1969), Atwood (1973), and Atwood and Sandeman (1982). Coelom and Blood-Vascular System The well-developed, metameric coelom characteristic of the annelids has undergone drastic reduction in the arthropods and is represented by only the cavity of the gonads and in certain arthropods by the excretory organs. The change is probably related to the shift from a fluid internal skeleton to a solid external skeleton. The other spaces of the arthropod body do not constitute a true coelom but rather a hemocoel—that is, merely spaces in the tissue filled with blood. Although derived from the annelids, the arthropod blood-vascular system is an open one. The dorsal vessel of annelids, which is contractile and the chief center for blood propulsion, may be homologous to the arthropod heart. The heart varies in position and length in different arthropodan groups, but in all of them the heart is a muscular tube perforated by pairs of lateral openings called ostia (Fig. 12-1A). Systole (contraction) results from the contraction of heart wall muscles, and diastole (expansion and filling) from suspensory elastic fibers and in some species from the contraction of suspensory muscles. The ostia enable the blood to flow into the heart during diastole from the large, surrounding sinus known as the pericardium. However, in arthropods the pericardium does not derive from the coelom, as in mollusks and vertebrates, but is a part of the hemocoel. After leaving the heart, blood is pumped out to the body tissues through arteries and is eventually dumped into sinuses (collectively the hemocoel) in which it bathes the tissues directly. The blood then returns by various routes to the pericardial sinus. The blood of arthropods contains several types of cells and in some species the respiratory pigment hemocyanin or, less commonly, hemoglobin. As in mollusks, arthropod hemocyanin is a large molecule dissolved in the plasma; however, the structure of arthropod hemocyanin indicates that it evolved independently from that of mollusks (see reviews by Mangum, 1985; Linzen et al, 1985). Arthropods possess two types of excretory organs. Malpighian tubules are blind tubules that lie within the hemocoel (blood-filled spaces) and open into the gut. Wastes pass from the blood into the tubules and then into the gut, where they are eliminated through the anus along with fecal material. Malpighian tubules are found in centipedes, millipedes, insects, and arachnids and represent an organ system that evolved independently within these groups or their arthropod ancestors. The other type of arthropod excretory organ is paired blind saccules that open by ducts to the outside of the body adjacent to an appendage (Fig. 129A). The excretory organ takes the name of the appendage with which it is associated—coxal glands, maxillary glands, etc. Since the saccule is derived embryonically from the coelom, the tubule may represent an old metanephridium that originally drained the coelom. Typically, the saccule wall is composed of podocytes (Fig. 12-9B) and is the site of filtration from the surrounding blood. Parts of the tubule may be modified for selective reabsorption and secretion. Although such paired excretory organs may be derived from nephridia, no living arthropod has more than a few such saccules.
The arthropod gut differs from that of most other animals in having large stomodcal and proctotj regions (Fig. 12-1A). The derivatives of thai todermal portions are lined with chitin and соиі tute the foregut and hindgut. The intervenioil gion, derived from endodcrm, forms the midpt The foregut is chiefly concerned with ingest* trituration, and storage of food; its parts are J iously modified for these functions, depending the diet and mode of feeding. The midgut ill site of enzyme production, digestion, andafcJ tion; however, in some arthropods enzymes! passed forward and digestion begins in the fore Very commonly the surface area of the midgat I increased by outpocketings, forming pouchaJ large digestive glands. The hindgut functionsaB absorption of water and the formation of feces. |